The Numbers a Clinician Reads
Keep your waist under half your height. Two blood pressure numbers where the lower one can decide it. And a measure that exists for dosing, not fitness.
The tools in this guide
All free, no signup. Each one has a printable sheet too.
- Blood Pressure Category CalculatorWhere a reading falls on the American Heart Association's categories - and which of your two numbers is the one deciding it.
- Waist to Height Ratio CalculatorKeep your waist to less than half your height. The ratio, the bands, and the actual waist measurement each band means for you.
- Body Surface Area CalculatorMosteller and Du Bois side by side, because they disagree - and the gap is the reason this is a clinician's number rather than yours.
These three are not fitness numbers. They are measures a clinician computes about you — two for screening, one for dosing — and each has a rule that is routinely misread.
⚠️ None of them is a diagnosis, and none should be acted on alone. They exist to tell you what to ask about.
Keep your waist under half your height
The Waist to Height Ratio Calculator carries the simplest rule in the whole subject, and it needs no chart.
A 90 cm waist at 175 cm tall:
| Ratio | 0.514 |
| Half your height | 87.5 cm |
| How far over that you are | 2.5 cm |
| Top of the healthy band (0.49) | 85.8 cm |
| To reach the healthy band | 4.3 cm |
"Keep your waist under half your height" is the whole guideline, and it works at any height without a table — which is exactly why it has held up as a screening measure.
Its advantage over BMI is that it looks at where weight sits rather than only how much there is. Weight around the middle behaves differently from weight elsewhere, and a single ratio catches that while a height-and-weight figure cannot.
⚠️ Measure it properly or the number means nothing. Bare skin, midway between the lowest rib and the top of the hip bone, tape horizontal, at the end of a normal breath out — and do not pull it tight. Two centimetres of measuring technique is the entire width of the band you are trying to place yourself in.
Two numbers, and the lower one can decide it
The Blood Pressure Category Calculator places a reading on the American Heart Association's table — and shows what each number says on its own, because the rule most people miss is that the worse of the two wins.
| Category | Systolic | Diastolic | |
|---|---|---|---|
| Normal | under 120 | and | under 80 |
| Elevated | 120–129 | and | under 80 |
| Stage 1 | 130–139 | or | 80–89 |
| Stage 2 | 140+ | or | 90+ |
"And" becomes "or" at Stage 1. So 118 over 85 is Stage 1 — an entirely ordinary top number and a bottom number that has crossed the line. Most people read the first number and stop.
⚠️ Over 180, or over 120, is a hypertensive crisis — not a worse Stage 2. Wait five minutes, measure again, and contact a doctor immediately if it stays there. With chest pain, shortness of breath, numbness, difficulty speaking or a change in vision, call emergency services rather than re-measuring.
One reading is never a diagnosis. Blood pressure moves with activity, caffeine, pain, a full bladder and simply being in a clinic — which is why home readings over a fortnight are worth more to a clinician than one number in a waiting room.
The one that is not about you at all
The Body Surface Area Calculator is the odd one here, and worth understanding precisely because it is not a health indicator.
For 170 cm and 70 kg:
| Mosteller | 1.82 m² |
| Du Bois | 1.81 m² |
| Difference | 0.46% |
BSA exists mainly for dosing — chemotherapy and some other drugs are prescribed per square metre — and for indexing measurements like cardiac output. There is no "good" or "bad" BSA. It is a scaling factor.
The useful thing the tool shows is that the two standard formulas agree to within half a percent at ordinary sizes. Which formula you use does not matter at this scale, and knowing that saves worrying about a choice that makes no difference.
What they have in common
| Measures | For | |
|---|---|---|
| Waist to height | where weight sits | screening |
| Blood pressure | pressure in your arteries | screening |
| Body surface area | your size, as a scaling factor | dosing |
The screening pair are both thresholds you can act on — and both have a technique problem, where how you measure moves the result by about the width of a category.
The dosing one is a reminder that not every number computed about your body is a verdict on it.
What to actually do
- Measure your waist properly and compare it to half your height. It takes a minute and needs no equipment.
- If you are tracking blood pressure, take two or three readings a minute apart, average them, and write them down for a fortnight.
- Take the sheet, not the calculator, to whoever you see.
A clinician can do far more with two weeks of properly-taken home readings than with anything a web page tells you — and that, rather than a category label, is what these tools are actually for.
General information, not medical advice. For anything that concerns you, speak to a doctor or pharmacist.
Get the next guide.
New tools and guides straight to your inbox. No spam, ever.